The invention relates to improving the switching reliability of a magnetic memory cell in a magnetic random access memory (MRAM). Embodiments of the invention add an antiferromagnet to a magnetic memory cell. An antiferromagnetic layer can be formed adjacent to a soft layer in the MRAM on a side of the soft layer that is opposite to a hard layer of the MRAM. One embodiment further includes an additional interlayer of non-antiferromagnetic material between the antiferromagnetic layer and the soft layer.
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1. A digital system comprising:
a magnetic random access memory (MRAM) configured to store data in antiferromagnetically stabilized pseudo spin valves (aspsvs), where an aspsv further comprises:
a hard layer of ferromagnetic material, where the hard layer is adapted to store data in a magnetic orientation;
a spacer layer of non-ferromagnetic material disposed adjacent the hard layer;
a soft layer of ferromagnetic material disposed adjacent the spacer layer such that the spacer layer is between the hard layer and the soft layer, where the soft layer is adapted to switch magnetic orientation to allow data to be read from the hard layer;
an antiferromagnetic layer disposed on a side of the soft layer that is opposite to the spacer layer; and
an afm interlayer disposed between the soft layer and the antiferromagnetic layer, where the afm interlayer is not formed from an antiferromagnetic material.
2. The digital system as defined in
3. The digital system as defined in
4. The digital system as defined in
5. The digital system as defined in
6. The digital system as defined in
8. The digital system as defined in
9. The digital system as defined in
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This application is a divisional application of U.S. application Ser. No. 10/760,127, filed Jan. 16, 2004, now U.S. Pat. No. 6,903,399, which is a continuation application of U.S. application Ser. No. 10/193,458, filed Jul. 10, 2002, now U.S. Pat. No. 6,707,084, which claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 60/354,623, filed Feb. 6, 2002, the disclosures of which are hereby incorporated by reference in their entireties herein.
This application is also related to U.S. application Ser. No. 11/103,347, filed Apr. 11, 2005, the disclosure of which is hereby incorporated by reference in its entirety herein.
This invention was made with Government support under Contract Number MDA972-98-C-0021 awarded by DARPA. The Government has certain rights in the invention.
1. Field of the Invention
The invention generally relates to memory technology. In particular, the invention relates to non-volatile magnetic memory.
2. Description of the Related Art
Computers and other digital systems use memory to store programs and data. A common form of memory is random access memory (RAM). Many memory devices, such as dynamic random access memory (DRAM) devices and static random access memory (SRAM) devices are volatile memories. A volatile memory loses its data when power is removed. For example, when a conventional personal computer is powered off, the volatile memory is reloaded through a boot up process. In addition, certain volatile memories such as DRAM devices require periodic refresh cycles to retain their data even when power is continuously supplied.
In contrast to the potential loss of data encountered in volatile memory devices, nonvolatile memory devices retain data for long periods of time when power is removed. Examples of nonvolatile memory devices include read only memory (ROM), programmable read only memory (PROM), erasable PROM (EPROM), electrically erasable PROM (EEPROM), flash memory, and the like. Disadvantageously, conventional nonvolatile memories are relatively large, slow, and expensive. Further, conventional nonvolatile memories are relatively limited in write cycle capability and typically can only be programmed to store data about 10,000 times in a particular memory location. This prevents a conventional non-volatile memory device, such as a flash memory device, from being used as general purpose memory.
An alternative memory device is known as magnetoresistive random access memory (MRAM). An MRAM device uses magnetic orientations to retain data in its memory cells. Advantageously, MRAM devices are relatively fast, are nonvolatile, consume relatively little power, and do not suffer from a write cycle limitation. A pseudo spin valve (PSV) MRAM device uses an asymmetric sandwich of the ferromagnetic layers and metallic layer as a memory cell, and the ferromagnetic layers do not switch at the same time.
The asymmetric sandwich of a PSV MRAM includes a “hard layer” that stores data and a “soft layer” that switches or flips to allow data to be stored and read in the hard layer. When operating as intended, the soft layer switches before the hard layer. The earlier switching of the soft layer advantageously inhibits switching of the hard layer, which then results in a higher write threshold for a PSV MRAM than for a spin valve MRAM.
One problem with conventional PSV MRAM devices is that the magnetization of the soft layer is not well controlled. A soft layer that fails to switch at a relatively low applied magnetic field can result in a PSV MRAM device that undesirably behaves as a spin valve rather than a PSV. This reduces the write threshold and can result in corrupting the stored data during a read operation. To protect PSV MRAM devices from data corruption, the fields generated during read operations are maintained to relatively low levels, which results in relatively low repeatability and cyclability of writing to and reading from memory cells.
Embodiments of the invention solve these and other problems by stabilizing the soft layer of a pseudo spin valve (PSV). Embodiments of the invention include a layer of antiferromagnetic material (AFM), which stabilizes the magnetization of the thin layer. The stabilization of the soft layer of the PSV provides PSV MRAM devices with relatively good repeatability and cyclability.
Embodiments of the invention include an antiferromagnet in a magnetic memory cell. An antiferromagnetic layer can be formed adjacent to a soft layer in an MRAM on a side of the soft layer that is opposite to a hard layer of the MRAM. One arrangement further includes an additional interlayer of non-antiferromagnetic material between the antiferromagnetic layer and the soft layer.
The antiferromagnetic material (AFM) is formed adjacent to or near to the soft layer of the PSV. The layer of AFM should be formed on a side of the soft layer that is opposite to a side with a hard layer of the PSV. In addition, an amount of coupling between the soft layer and the AFM layer should be sufficiently low enough to allow the soft layer to switch at a lower magnetic field than the hard layer, thereby maintaining a relatively wide spread between the strength of a magnetic field used in a read operation and the strength of a magnetic field used in a write operation.
One embodiment of the invention includes an antiferromagnetically stabilized pseudo spin valve (ASPSV) in a magnetic random access memory (MRAM). The ASPSV includes a hard layer of ferromagnetic material, a soft layer of ferromagnetic material, a spacer layer of non-ferromagnetic material disposed between the hard layer and the soft layer; and an antiferromagnetic layer disposed adjacent to the soft layer. The antiferromagnetic layer should also be disposed on a side of the soft layer that is opposite to the hard layer. The antiferromagnetic layer can be formed from an alloy of manganese, such as from ferro manganese (FeMn).
Another embodiment of the invention includes an antiferromagnetically stabilized pseudo spin valve (ASPSV) in a magnetic random access memory (MRAM) with an AFM interlayer. The ASPSV includes a hard layer of ferromagnetic material adapted to store data in a magnetic orientation, a soft layer of ferromagnetic material adapted to switch orientation to allow data to be read from the hard layer, a spacer layer of non-ferromagnetic material disposed between the hard layer and the soft layer, an antiferromagnetic layer disposed on a side of the soft layer that is opposite to the hard layer, and the AFM interlayer. The AFM interlayer is disposed between the soft layer and the antiferromagnetic layer. The AFM interlayer can be formed from a variety of materials, but should not be formed from an antiferromagnetic material. Suitable materials for the AFM interlayer include iridium (Ir), copper (Cu), ruthenium (Ru), chromium (Cr), and aluminum (Al). The AFM interlayer can be relatively thin, such as about a monolayer in thickness.
Another embodiment of the invention includes a method of stabilizing a pseudo spin valve (PSV). The method includes providing a magnetoresistive sandwich that includes a soft layer and a hard layer, and forming an antiferromagnetic layer on the magnetoresistive sandwich near to the soft layer and on a side of the soft layer that is opposite to the hard layer. The antiferromagnetic layer can be formed adjacent to the soft layer, or a AFM interlayer can also be formed between the soft layer and the antiferromagnetic layer.
These and other features of the invention will now be described with reference to the drawings summarized below. These drawings and the associated description are provided to illustrate preferred embodiments of the invention and are not intended to limit the scope of the invention.
Although this invention will be described in terms of certain preferred embodiments, other embodiments that are apparent to those of ordinary skill in the art, including embodiments that do not provide all of the benefits and features set forth herein, are also within the scope of this invention. Accordingly, the scope of the invention is defined only by reference to the appended claims.
A magnetoresistive random access memory (MRAM) stores data in magnetic states of its memory cells. The electrical resistance of the cell varies depending on the stored magnetic state of the cell. The stored state of the cell is detected by sensing the difference in resistance.
To read data from the GMR cell 100, currents are again applied to the word line 102 and the bit line 104 corresponding to the GMR cell 100. The resistance encountered by the current applied to the bit line 104 varies depending on the logical state stored in the magnetic layers. A cell with a larger resistance exhibits a larger voltage drop with the current than a cell with a smaller resistance.
The GMR cell 300 stores data as a magnetic orientation in the second magnetic layer 310. A relatively high magnetic field is required to switch the magnetization of the second magnetic layer 310 so that the magnetization remains fixed in operation. The magnetic state of the GMR cell 300 is switched by switching the magnetization of the first magnetic layer 306, which can be switched with a relatively low magnetic field generated by applying current to the corresponding word line 302 and the corresponding bit line 304. The resulting magnetization of the first magnetic layer 306 is either parallel or anti-parallel to the magnetization of the second magnetic layer 310. When the magnetization in the first magnetic layer 306 is parallel with the magnetization of the second magnetic layer 310, the electrical resistance of the GMR cell 300 is lower than when the magnetization is relatively is anti-parallel. Current in the word line 302 and/or the bit line 304 can be switched in both directions to correspondingly switch the magnetization of the first magnetic layer 306, i.e., the soft magnetic layer, between parallel and anti-parallel states. The difference in electrical resistance of the bit line 304 is then sensed, thereby allowing the stored logical state of the GMR cell 300 to be retrieved.
The illustrated magnetoresistive stack 400 includes an underlayer 402, a hard layer 404, a first interlayer 406, a spacer layer 408, a second interlayer 410, a soft layer 412, an antiferromagnetic (AFM) layer 414, a first cap layer 416, and a second cap layer 418. The underlayer 402 or seeding layer provides adhesion between an underlying layer in the substrate and the hard layer 404 by providing texture to the stack. The underlayer 402 can also protect against the undesired diffusion of atoms from the hard layer 404 to an underlying layer, such as a silicon substrate. A variety of materials can be used for the underlayer 402. In one embodiment, the underlayer 402 is formed from tantalum (Ta). Other materials that can be used for the underlayer 402 include titanium (Ti), ruthenium (Ru), nickel iron chromium (NiFeCr), and tantalum nitride (TaN). The underlayer 402 can be formed to a broad range of thicknesses. In one embodiment, the underlayer 402 is within a range of about 10 Angstroms (Å) to about 100 Å thick. Various processing techniques, such as physical vapor deposition (PVD) techniques, chemical vapor deposition (CVD) techniques, and the like, can be used to form the various layers described herein.
The hard layer 404 (or thick layer) stores the data for the antiferromagnetically stabilized PSV cell. A relatively large word current, which generates a relatively large magnetic field, switches the orientation of the magnetic moment stored in the hard layer 404 to store data. The hard layer 404 can be made from a variety of ferromagnetic materials, such as permalloy (Ni80Fe20), cobalt-iron (Co90Fe10), and the like. In one embodiment, the hard layer 404 is within a range of about 20 Å to about 100 Å thick.
The first interlayer 406 is optional. The first interlayer 406 can be included in the magnetoresistive stack 400 to enhance the signal, i.e., the change in resistance, from the magnetoresistive stack 400. In one embodiment, where the hard layer 404 is formed from permalloy, the first interlayer 406 is formed from cobalt (Co) or from an alloy that includes cobalt, such as Co90Fe10, Co80Fe20, and the like. In one example, the thickness of the first interlayer 406 is within a range of about 2 Å to about 15 Å.
The spacer layer 408 is a nonmagnetic layer that separates the magnetic layers. The spacer layer 408 can be formed from a broad variety of non-ferromagnetic materials. A broad variety of materials can be used to form the spacer layer 408. In one embodiment, the spacer layer 408 is copper (Cu). Alloys of copper are also suitable materials, such as copper silver (CuAg), copper gold silver (CuAuAg), and the like. In one example, the thickness of the spacer layer 408 is within a range of about 18 Å to about 45 Å.
The second interlayer 410 is optional. The second interlayer 410 can be included to enhance the signal from the magnetoresistive stack 400. In one embodiment, where the soft layer 412 is formed from permalloy, the second interlayer 410 is formed from cobalt (Co) or from an alloy that includes cobalt, such as Co90Fe10, Co80Fe20, and the like. The thickness of the second interlayer 410 can correspond to a range of about 2 Å to about 15 Å.
The magnetic moment of the soft layer 412 (or thin layer) can be switched or flipped with relatively low word currents and relatively low magnetic fields. When the magnetic moment of the soft layer 412 and the magnetic moment of the hard layer 404 are parallel, the resistance of the PSV cell is relatively low. When the magnetic moment of the soft layer 412 and the magnetic moment of the hard layer 404 are anti-parallel, the resistance of the PSV cell is relatively high. The soft layer 412 can be made from a variety of materials, such as permalloy (Ni80Fe20), cobalt-iron (Co90Fe10), and the like. In one embodiment, the thickness of the soft layer 412 is about 20% to about 80% thinner than the thickness of the hard layer 404.
The AFM layer 414 is a layer of an antiferromagnetic material. An antiferromagnetic material produces anti-parallel alignments of electron spins in response to an applied magnetic field and has no net magnetic moment. The AFM layer 414 assists to control the magnetization of the soft layer 412 such that the soft layer 412 more consistently switches magnetic moments at a relatively low applied magnetic field, thereby allowing the antiferromagnetically stabilized PSV MRAM to maintain relatively safe and robust high write thresholds, i.e., improves the switching reliability of the thin layer. Electrically, the AFM layer 414 is in parallel with the soft layer 412. This can disturb the detection of the variable resistance from the soft layer 412, which is used to detect the memory state stored in the hard layer 404. To reduce the disturbance to the detection of the memory state, the AFM layer 414 should be formed from a material with a relatively high resistivity and/or should be relatively thin.
The AFM layer 414 is preferably formed from an alloy of manganese, such as an antiferromagnetic alloy of ferro manganese (FeMn) including Fe50Mn50. Other suitable alloys of manganese include iridium manganese (Ir20Mn80), platinum manganese (PtMn), and nickel manganese (Ni45Mn55). The AFM layer 414 can also be formed from an oxide of a ferromagnetic material, such as nickel oxide (NiO) and nickel cobalt oxide (NiCoO), and the like, but such oxides can be relatively unstable over temperature. The AFM layer 414 should be disposed on a side of the soft layer 412 that is opposite to the hard layer 404. In addition, the AFM layer 414 should not be so thick that pinning of the soft layer results, which detrimentally results in spin valve characteristics from the pseudo spin valve. In one embodiment, the thickness of the AFM layer 414 is within a range of about 10 Å to about 70 Å. The thickness of the AFM layer 414 can vary according to the thickness and the switching fields of the hard layer 404 and the soft layer 412.
The first cap layer 416 (or protective cap layer) provides adhesion to the AFM layer 414 and provides a barrier against the undesired diffusion of atoms from the AFM layer 414 to other layers in the substrate assembly. In one embodiment, the first cap layer 416 is formed from tantalum (Ta). Other materials that can be used for the first cap layer 416 include copper (Cu), titanium nitride (TaN), and the like. The thickness of the first cap layer 416 can vary in a broad range. In one embodiment, the thickness of the first cap layer 416 is within about 50 Å to about 500 Å thick.
The second cap layer 418 (or diffusion barrier cap layer) is an optional layer. For some etching processes, the addition of the second cap layer 418 provides a relatively good stopping layer. In one embodiment, the second cap layer 418 is a layer of chromium silicon (CrSi). Other materials that can be used for the second cap layer 418 include copper (Cu), tantalum (Ta), titanium nitride (TiN), and the like. In one embodiment, the thickness of the second cap layer 418 is within a range of about 100 Å to about 200 Å thick, but it will be understood by one of ordinary skill in the art that the thickness can vary within a broad range.
The AFM layer 504 can be formed from an antiferromagnetic material, such as ferro manganese (FeMn). Other materials that are suitable for the AFM layer 504 include various other alloys of manganese as well as various oxides as described earlier in connection with
The AFM interlayer 502 is disposed between the AFM layer 504 and the soft layer 412. In one example, the AFM interlayer 502 has a thickness within a range of about 1 Å to about 5 Å. Preferably, the AFM interlayer 502 is about a monolayer in thickness, i.e., about one atomic layer thick. In one embodiment, the AFM interlayer 502 is less than a monolayer in thickness. The AFM interlayer 502 serves as a spacer layer between the AFM layer 504 and the soft layer 412.
Advantageously, the AFM interlayer 502 can be used to adjust or to select the amount of coupling between the AFM layer 504 and the soft layer 412 by reducing the coupling strength between the AFM layer 504 and the hard layer 404 and/or the soft layer 412. However, the AFM interlayer 502 should not be so thick that coupling between the AFM layer 504 and the soft layer 412 is lost. Further advantageously, the AFM interlayer 502 can also enhance the uniformity of the coupling between the AFM interlayer 502 and the soft layer 412.
As illustrated in
The horizontal or x-axis represents the first H-field that is swept along one axis of the magnetoresistive stack 400. When the magnetoresistive stack 400 is subjected to the second H-field, the magnetoresistive stack 400 switches for a write when the magnitude of the first H-field is about 53 Oe. This is lower than the approximately 75–80 Oe described in connection with
The magnetic orientation of the soft layer 412 of the magnetoresistive stack 400 is switched or flipped in both directions, and the difference in resistance is interrogated to read the value of the data stored in the magnetoresistive stack 400. The R-H plot of
Various embodiments of the invention have been described above. Although this invention has been described with reference to these specific embodiments, the descriptions are intended to be illustrative of the invention and are not intended to be limiting. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined in the appended claims.
Katti, Romney R., Drewes, Joel A., Vogt, Timothy J.
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